1. Field of the Invention
The present invention relates to image capturing apparatuses and methods.
2. Description of the Related Art
Known examples of an image capturing apparatus with an automatic focusing mechanism include image capturing apparatuses that include, in addition to an optical system (hereinafter, “first optical system”) for obtaining an object image, another optical system (hereinafter, “second optical system) for measuring a distance to an object to be photographed by triangulation. Each of the first optical system and the second optical system includes a lens; the lenses are located away from each other on the image capturing apparatus. Parallax between the two lenses located away from each other causes an object image obtained through one of the two lenses to differ from that obtained through the other lens. In particular, when a distance between the object and the image capturing apparatus is small, there can be a case that although an object image obtained through the lens of the first optical system is at a center of an angle of view, an object image obtained through the lens of the second optical system is displaced from the center of the angle of view. There can be even a case where the object image obtained through the lens of the second optical system falls out of the angle of view.
An effect of the parallax is described below with reference to FIGS. 11A to 11D. FIG. 11A schematically illustrates an image capturing apparatus 100 directed toward an object X1 as viewed from a back side of the apparatus. The object X1 is displayed on a display unit 120 provided on the back side of the image capturing apparatus 100. Referring to FIG. 11A, a rectangular area (a range-finding area 400) displayed on a center portion of the object X1 is an image data area to be used in computation for measuring a distance to the object. The range-finding area 400 can be determined according to a point on the display unit 120 touched by a photographer, or, alternatively, a preset area.
The relationship between the first optical system and the second optical system is further described below. The second optical system is tilted so that, for instance, the range-finding area 400 is close to a center of an object image (close to a center of the angle of view of the first optical system) when located at a distance La from the object as illustrated in FIG. 11B. When the second optical system is at a distance Lb (La<Lb) from the object, as illustrated in FIG. 11C, the range-finding area 400 is displaced to a lower-left direction on the display unit 120. When the object X1 is photographed by zooming in at the distance Lb to the object X1, the range-finding area 400 is further displaced. The image capturing apparatus 100 calculates the distance to the object using image data in the range-finding area 400. Accordingly, when the range-finding area 400 is displaced from an object to be focused, photographing in desired focus cannot be achieved.
An image capturing apparatus that determines a distance to an object by the triangulation has a parallax-related problem as described above. Difference between the angle of view of the first optical system and that of the second optical system is adjusted at a manufacturing stage of the image capturing apparatus and cannot be adjusted when taking photographs. An image capturing apparatus with a high-magnification zoom function calculates a focal length by a different method than the triangulation described above because an influence of the parallax is likely to be more serious in such an image capturing apparatus than in an image capturing apparatus without a high-magnification zoom function. An influence of the parallax cannot be removed, either, in an image capturing apparatus with interchangeable lenses in which a mechanism such as hill-climbing autofocusing (AF) method, which is different from the triangulation, is adopted for automatically focusing on an object.
The hill-climbing AF method is a method for determining an in-focus position as a position at which a contrast value, obtained by computation on image data while gradually moving the lens of the first optical system, is maximized. However, the hill-climbing AF method involves a motion of a picture-taking lens; accordingly, when the picture-taking lens has a long focal length, a traveling distance of the picture-taking lens becomes long, resulting in relatively long autofocusing time.
Under such circumstances, image capturing apparatuses that perform autofocusing by using a hybrid method, which is a combination of the triangulation and the hill-climbing AF method, have been proposed (see, for instance, Japanese Patent Application Laid-open No. 2001-221945 and Japanese Patent Application Laid-open No. 2006-072384).
In the hybrid methods, a picture-taking lens is moved to a position in accordance with an object distance measured by the triangulation; thereafter, the hill-climbing AF method is used to move the picture-taking lens, within a predetermined range, in a vicinity of the position to which the picture-taking lens has been moved. The hybrid method allows, even when there is a certain degree of parallax, an in-focus position to be determined accurately by the hill-climbing AF method. This method also shortens the moving distance of the picture-taking lens and hence saves time required for autofocusing because the hill-climbing AF method is performed in the predetermined moving range after an approximate in-focus position has been determined by the triangulation.
The apparatus disclosed in Japanese Patent Application Laid-open No. 2001-221945 changes an AF area for use in the hill-climbing AF method in accordance with multi-point focusing areas determined by an AF unit. The apparatus disclosed in Japanese Patent Application Laid-open No. 2006-072384 switches an AF area for use in the hill-climbing AF method in accordance with a distance to an object. In each of these apparatuses, the range-finding area to be focused in the hill-climbing AF method needs to be an area corresponding to a range-finding angle of view of the second optical system having a fixed focal point.
As described above, an image capturing apparatus that measures a distance to an object by the triangulation determines a focal point less accurately due to parallax between an image-capturing optical system and an AF optical system. Some type of recent models of image capturing apparatuses displays an object image on a display unit to allow a photographer to designate a to-be-focused position on the display unit. Other type is configured to automatically identify a face in an object image and automatically designates a position of the face as a range-finding area. The object image displayed on the display unit is obtained with an image-capturing optical system. Hence, an image capturing apparatus, that is capable of correcting an influence of parallax in calculation of a focal length by using an AF optical system and obtaining a more accurate focal length irrespective of what range-finding area is designated in an object image obtained with an image-capturing optical system, has been desired.